This invention relates to methods for producing fused silica glass and, in particular, to methods for obtaining desired levels of dissolved hydrogen in high purity fused silica (HPFS) glass.
Dissolved molecular hydrogen in HPFS glass is required to provide the glass with resistance to structural damage when exposed to laser pulses. One method for producing HPFS glass, which is in commercial use by the assignee of this invention, involves introducing an organic feedstock containing silicon along with natural gas and oxygen to burners located in the crown of a high temperature refractory furnace. The burners produce fine silica particles (soot particles) which are collected to form a glass boule. Preferred raw materials are natural gas as the fuel and octamethylcyclotetrasiloxane (OMCTS) as the silicon source, both of which are burned in the presence of oxygen to produce the soot particles.
The combustion of the feedstock and the fuel also produces hydrogen either as an intermediate combustion product or from the dissociation of water molecules. Thus, in the burner flame, an atmosphere containing hydrogen surrounds the silica particles. Since the flame temperatures are high and hydrogen is soluble in silica, there is always an equilibrium between dissolved hydrogen in silica and the amount of hydrogen in the atmosphere surrounding the silica particles.
FIG. 1 is a schematic diagram of a furnace that can be used to produce silica-containing bodies (boules) in accordance with the above technique. As shown therein, furnace 10 includes crown 11 and side walls 12 which together form furnace cavity 13. Located within the cavity is cup 14, which has a bottom surface 15 and side walls 16 for, respectively, supporting and retaining the growing boule. Bottom surface 15 is typically covered with a sand bait at the beginning of the soot deposition process. The furnace can also include internal wall 17 which moves with cup 14 and controls air flow around the boule as it is deposited. See commonly-assigned PCT Patent Publication No. WO97/10182, the contents of which are incorporated herein by reference.
As indicated by arrows 18 and 19 in FIG. 1, the design of the furnace is such that room air is infiltrated into the furnace cavity 13 either through the burner holes 20 formed in the crown or though the gap between cup 14 and internal wall 17. The infiltrated air reduces the concentration of hydrogen in the furnace atmosphere in two ways. First, it physically dilutes the combustion gases, and second, it brings in additional oxygen that reacts with the hydrogen to form water vapor. The overall result of the entrained air is a reduction in dissolved hydrogen in the glass boule.
As practiced in the past, the above process has suffered from the problem of lack of control of the air that is infiltrated into the furnace cavity. While the flow of OMCTS, natural gas, and oxygen through the burners has been carefully regulated, there has been no control of the air infiltrating into the process. Such factors as cavity pressure or crown curvature that can influence the level of infiltrated air have not been monitored or controlled.
In accordance with the invention, it has been determined that this lack of control over infiltrated air has resulted in a process that is different between furnaces and different from run-to-run on the same furnace. The boules coming out of these furnaces not only have low dissolved hydrogen, but also have uncontrolled, variable amounts of dissolved hydrogen. Moreover, the problem is not detected until glass samples from the boules are core-drilled and analyzed for dissolved hydrogen. This core-drilling and analyzing process typically takes from days to weeks. If the boules do not meet the required lower limit on hydrogen, then parts made from the boules generally cannot be used in pulsed laser applications.
U.S. Pat. No. 5,719,698, assigned to Nikon Corporation, discloses a procedure for increasing the hydrogen content of silica glasses by supplying hydrogen to the burner used to produce the glass. In particular, the patent describes supplying hydrogen to the inner tubes of a burner and using hydrogen as a carrier gas for SiCl4 or SiHCl3. The patent contains no disclosure or suggestion that the dissolved hydrogen content of a glass boule can be controlled by monitoring the hydrogen concentration in a furnace cavity and then controlling the pressure within the cavity and/or flows to the furnace""s burners (e.g., flows of oxygen and/or natural gas) based on the thus monitored hydrogen concentration.
In view of the foregoing, it is an object of the present invention to provide improved methods for producing silica-containing bodies. More particularly, it is an object of the invention to provide improved methods for producing silica-containing bodies having desired levels of dissolved hydrogen. Preferably, the silica-containing body is high purity fused silica.
To achieve these and other objects, the invention provides a method for forming a silica-containing body comprising:
(a) providing a furnace (10) which comprises:
(i) a cavity (13);
(ii) at least one burner (21) which produces soot particles; and
(iii) a surface (15) within the cavity (13) for collecting the soot particles to form the body;
(b) collecting the soot particles to form the body;
(c) monitoring the hydrogen content within the cavity (13); and
(d) controlling the hydrogen content within the cavity (13) based on the monitoring of step (c).
In certain embodiments, the pressure within the cavity is controlled to achieve a predetermined hydrogen content in the cavity which correlates with a desired dissolved hydrogen content in the silica-containing body. In other embodiments, burner flows, either alone or in combination with cavity pressure, are used for this purpose.
Preferably, the hydrogen content in the cavity is monitored using a high temperature probe (22), a trap (23) for soot particles, and a gas chromatograph (24).
A particularly important advantage of the invention is that the required amount of dissolved hydrogen in the glass is achieved by adjusting the hydrogen in the furnace atmosphere right at the start of the glass-boule forming process. In this way, rejection of large quantities of glass for low levels of dissolved hydrogen is avoided.